(a) Field of the Invention
The present invention relates to an in-plane-switching-mode (IPS-mode) liquid crystal display (LCD) device and, more particularly, to an IPS-mode LCD device having a uniform electric field in each pixel. The present invention also relates to a method for manufacturing a LCD device.
(b) Description of the Related Art
The LCD devices are categorized into two modes including: a first mode wherein orientation of liquid crystal (LC) molecules in the LC layer is rotated in the direction normal to the surface of the substrates sandwiching therebetween the LC layer; and a second mode wherein orientation of LC molecules are rotated in the direction parallel to the surface of the substrates (substrate surface). The first modes include a twisted-nematic-mode (TN-mode) LCD device as a typical example, and the second modes include an IPS-mode LCD device as a typical example.
In the TN-mode LCD device, since the LC molecules are rotated and deviated from the plane normal to the substrate surface, the polarization angle of the light passed by the LC layer deviates along with the increase of the viewing angle to achieve only a limited viewing angle characteristic. On the other hand, in the IPS-mode LCD device, since the LC molecules are rotated in the plane normal to the substrate surface, polarization angle of the light does not change even with a larger viewing angle, to thereby achieve a wider viewing angle characteristic. Thus, an increased number of 10 IPS-mode LCD devices are employed.
It is noted here that the IPS-mode LCD devices include a first type wherein a pair of electrodes in each pixel are disposed on the common surface of an insulation film, and a second type wherein the pair of electrodes in each pixel are disposed on opposite surfaces of an insulation film. The second-type LCD device is shown in FIG. 7.
The LCD device of FIG. 7 includes a thin-film-transistor (TFT) substrate 10 disposed on the light-incident side of the LC layer 30 and a counter substrate 20 disposed on the light 20 emitting side of the LC layer 30. The TFT substrate 10 includes a passivation film 17 having a bottom surface on which a common electrode 19 is formed and a top surface on which a pixel electrode 16 is formed.
FIG. 8A shows an arrangement of the pixel electrode 16 25 and common electrode 19 in a pixel of the LCD device of FIG. 7. FIG. 7 corresponds to a sectional view taken along line VII-VII in FIG. 8A. The pixel electrode 16 and common electrode 19 oppose each other in the lateral direction of the pixel in FIG. 8A. Since the pixel electrode 16 and common electrode 19 sandwich therebetween the passivation film 17, as described above, both the electrodes 19 and 16 may overlap each other as viewed normal to the substrate surface.
FIG. 8B shows a pixel of the first-type IPS-mode LCD device, generally designated by numeral 201, wherein both the pixel electrode 16 and common electrode 19 are formed on the common surface of an insulation film. In the structure of FIG. 8B, the pixel electrode 16 and common electrode 19 have therebetween a specific gap as viewed normal to the substrate surface for avoiding a short-circuit failure
The structure of FIG. 8A has an advantage that the pixel electrode 16 has a larger capacitance with respect to the common electrode 19, which improves the charge storage capability thereof. The structure of FIG. 8A has another advantage that the lateral electric field applied to the LC layer increases in the vicinity of the edge of the electrodes 16, 19, thereby increasing a light transmission factor during display of a bright state.
The arrangement of the LCD device such as shown in 25 FIGS. 7 and 8A is described in Patent Publication JP-2004-62145A, for example.
In the LCD device 200 of FIG. 7, the passivation film 17 should have an insulation function sufficient to withstand the voltage between the pixel electrode 16 and the common electrode 19. If the thickness of the passivation film 17 has a wide range of variation, and thus has a thickness in a pixel or pixels below a required range for withstanding a voltage applied between the pixel electrode 16 and the common electrode 19, a breakdown of the passivation film 17 will result in the pixel or pixels suffering a defect known as “a point defect” in the LCD device.
On the other hand, if the passivation film 17 has a thickness greater than the required range, the voltage to be applied to the LC layer 30 is reduced by the passivation films 17, whereby the passivation film 17 may prevent a sufficient lateral electric field from being applied to the LC layer 30. For avoiding such a scenario to impact the lateral electric field, higher voltage must be applied between the pixel electrode 16 and the common electrode 19, which increases the power dissipation of the LCD device.
In addition, the passivation film 17 having a larger thickness provides a wider range of variation in the lateral electric field applied to the LC layer 30. This variation causes a wider range of variation in the storage function for the electric charge among the pixels, which may create a flicker or residual image in the LCD device.